1. Field of the Invention
The present invention relates to an evaporation donor substrate used for deposition of materials that can be deposited by an evaporation method, a method for manufacturing the evaporation donor substrate, and a method for manufacturing a light-emitting device using the evaporation donor substrate.
2. Description of the Related Art
Light-emitting elements using an organic compound as a light emitter, which are characterized by thinness, lightweight, fast response, and direct current low voltage driving, have been expected to be applied to next-generation flat panel displays. In particular, display devices in which light-emitting elements are arranged in matrix have been considered to be superior to conventional liquid crystal display devices in terms of a wide viewing angle and good visibility.
It is considered that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with an EL layer interposed therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light emission center of the EL layer to form molecular excitons, and energy is released to emit light when the molecular excitons relax to the ground state. A singlet excited state and a triplet excited state are known as the excited states, and it is thought that light emission can be obtained through either of the excited states.
An EL layer included in a light-emitting element has at least a light-emitting layer. In addition to the light-emitting layer, the EL layer can have stacked layers of structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and the like.
EL materials for forming an EL layer are broadly classified into a low molecular (a monomer) material and a high molecular (a polymer) material. In general, a low molecular material is often deposited by an evaporation method and a high molecular material is often deposited by ink-jet or the like.
An evaporation apparatus that is used in an evaporation method includes a substrate holder onto which a substrate is mounted; a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material; a heater for heating the EL material in the crucible; and a shutter for preventing the EL material from being scattered during sublimation. The EL material that is heated by the heater is sublimed and deposited on the substrate.
Note that in order to achieve uniform deposition, actually, a deposition target substrate needs to be rotated and the substrate and the crucible need to be separated from each other by at least a certain distance. In addition, when films of different colors are separately formed using a plurality of EL materials through a mask such as a metal mask, it is necessary that the distance between pixels be designed to be large and that the width of a partition wall (bank) formed of an insulator between the pixels be large. Such needs are major problems in advancing miniaturization of display pixel pitches along with higher definition (an increase in the number of pixels) and reduction in size of a light-emitting device including a light-emitting element.
Therefore, in order to achieve higher definition and higher reliability of flat panel displays, it has been required to solve those problems as well as to achieve high productivity and cost reduction.
Thus, a method for forming an EL layer of a light-emitting element through laser thermal transfer has been proposed (see Reference 1: Japanese Published Patent Application No. 2006-309995). Disclosed in Reference 1 is a transfer substrate in which a photothermal conversion layer including a low reflective layer and a high reflective layer and a transfer layer are provided over a supporting substrate. Irradiation of such a transfer substrate with laser light allows the transfer layer to be transferred to an element forming substrate.
However, in the transfer substrate of Reference 1, the high reflective layer and the low reflective layer are stacked on one side of the substrate. Therefore, even with the use of the high reflective layer, some heat absorption cannot be avoided. Thus, when the energy of the laser light is large, not only a portion of the transfer layer over the low reflective layer but also a portion of the transfer layer over the high reflective layer may be transferred.
In addition, in the structure illustrated in FIG. 3 of Reference 1, as also described in paragraph [0041], the low reflective layer and the high reflective layer must be formed with no space therebetween, which requires highly accurate patterning.
Furthermore, in the structure illustrated in FIG. 7 of Reference 1, the low reflective layer is patterned, the high reflective layer is then formed over the entire surface, and after that, the transfer layer is formed. In this structure, heat from the low reflective layer that is heated by absorbing laser light is transferred to the transfer layer through the high reflective layer. Thus, not only a desired portion of the transfer layer but also a portion of the transfer layer around the desired portion may be transferred.